1. Field of the Invention
The present invention relates to an optical bus type communication system which is suitable for the ISDN user network and local area network.
2. Description of the Related Art
In a local area network, an optical communication is employed and is put into practical use in order to cope with requirements in high speed and large capacity communication for increased amounts of information.
The ISDN user network which is widely investigated now is in the trend of employing an optical communication in order to realize higher and larger capacity communication.
An optical bus type communication system is typical as the system structure to realize such requirement. FIG. 1 indicates a system structure in case such an optical bus type communication system is adopted into the ISDN user network.
In FIG. 1, an optical signal sent from a user network interface 41 with the switching network is branched by optical couplers 21, 22, . . . , 2n inserted along an optical fiber 40 and is then distributed to terminals 1-3, 2-3, . . . , n-3.
The optical sending signals from respective terminals 1-3, 2-3, . . . , n-3 are combined by optical couplers 31, 32, . . . , 3n inserted on an optical fiber 20, transmitted to the user network interface 41, received by a data receiving part 42 and transmitted to switching equipment 46 through subscriber lines 45 passing the controller 44.
In this case, the optical sending signal from the furthest terminal 1-3 from the user network interface 41 reaches the user network interface 41 passing all optical couplers 31, 32, . . . , 3n and full length of the optical fiber 20. Therefore, it shows largest loss. Meanwhile the optical sending signal from the nearest terminal n-3 to the user network interface 41 reaches the interface passing only the optical coupler 3n and the optical fiber 20 up to the user network interface 41. Therefore it shows smallest loss.
The data receiving part 42 of user network interface 41 is provided with an automatic gain controlling function which makes small variation of output level due to input level difference, but in case optical output levels from respective terminals and coupling coefficients of the optical couplers 31, 32, . . . , 3n are equal, the response time of automatic gain controlling function becomes longer. Accordingly, discrimination error may be generated during this response time because optical receiving levels are remarkably different if, for example, a first signal is input from the nearest terminal n-3 and then another signal is input immediately after such first signal from the furthest terminal 1-3.
Therefore, optical couplers having different coupling coefficients are used or light emitting elements having different optical output levels are used, so that the optical sending signal levels from respective terminals are almost equal at the receiving point of the user network interface 31. However, this results in a problem in that difficult adjustment is still required.
Meanwhile, in such optical bus type communication system that the fixed channels are generally not assigned to respective terminals, and these terminals have one or a plurality of channels in common.
Accordingly, in some cases a plurality of terminals may simultaneously access the same channel. In this case, the optical signals transmitted from these terminals collide, a plurality of optical signals overlap, and thereby the optical receiving level at the data receiving part 42 within the user network interface 41 increases. As described previously, when collision of signals occurs, even if the optical receiving level of the data receiving part 42 is equalized in case each terminal solely transmits optical signal by adjusting optical couplers and light emitting elements of respective terminals, the optical signals from a plurality of terminals selected freely overlap and thereby the optical receiving level of data receiving part 42 varies.
Such a problem also occurs in the case of an optical LAN illustrated in FIG. 2 (Local Area Network). Namely, in the optical LAN, the user network interface 41 does not exist and communication between terminals is carried out by forming the U-shaped optical fiber with the optical fibers 40 and 20. In this case, a problem similar to that occurring in the data receiving part 42 in the user network interface 41 also occurs.
This problem also occurs in the optical LAN indicated in FIG. 3.
In the structure illustrated in FIG. 3, the main station 41' controls the right for making access to substations 1-3, n-3 (terminals). The main station 41' and a plurality of substations 1-n, n-3 are connected through optical couplers 21, 2n and 31, 3n provided on the two lines of optical fiber cables (or buses) 40', 20'. The optical coupler 2n on the down-line optical bus 40' branches the signal from the main station 41' and inputs this signal to the corresponding substation n-3 and causes the signal to pass through to the optical coupler 21 in the succeeding stage. The optical couplers 31, 3n on the up-line optical bus 20' also has similar function.
In this optical bus type communication system, since the signal is branched or inserted through the optical coupler as the bus access element for both up-and down-lines, the optical signal from the nearest substation n-3 of the up-line 20' is input to the main station 41' almost without transmission loss, but the optical signal from the furthest substation 1-3 is input to the main station 41' having suffered to transmission loss every time it passed an optical coupler. Therefore, a difference is present in the level of optical pulses received by the main station 41'. Moreover, in case competition occurs in optical pulses transmitted from the substations, the pulses overlap. As a result, the amplitude becomes equal to several times of the amplitude in the case when competition does not occur between the input pulse of the main station 41'. When a signal pulse having such a large level difference is input to the main station, the AGC (Automatic Gain Controller: not illustrated) in the main station 41' cannot follow the signal pulse, resulting in a problem in that the receiving data cannot be read.